How to stop your bathtub from overflowing
Venkatesh Srinivas (S2 '18) & Sid Shanker (S1'18)
TCP Goals
Sender
Sender
Router
Receiver
1 GB/s
1 GB/s
1 GB/s
Sender
Link
Packets waiting to send
Router
TCP: Goals
TCP tries to estimate the bandwidth of a link.
How big a window is "too big"?
Packet loss (bathtub overflowing)
Sending window over time
No active queue management
It turns out, managing the queue differently can help!
Introducing:
RED
Drop packets before the queue is full
Introducing:
RED
Drop packets before the queue is full
Maybe.
Packet
Packet
Packet :(
Packet
Packet
Packet
Packet
Packet
Packet
Packet
A queue using RED
With traditional droptail
With RED
But, sometimes, this would happen:
We only measure the average queue length when an item is added to the queue.
"What's an average?"
Turns out, the "spike" was having a HUGE impact on our average queue depth.
Solution: use time!
By decaying the average over time, we allow our algorithms to recover.
